1. Field of the Invention
The present invention relates to a reinforced plastic composite structure. More particularly, the present invention relates to a corrosion resistant plastic composite structure, such as pipe, which is not prone to failure when exposed to corrosive liquids, e.g. acid.
2. Description of Prior Art
Reinforced plastic composite structures, for example pipes, are commonly formed by winding resin-impregnated filaments on a removable mandrel, curing the resin-impregnated structure and subsequently removing the mandrel from the cured pipe. By appropriate selection of the filaments and the resin (typically glass filaments and a polyester resin), the composite pipe can be fabricated so as to exhibit generally acceptable corrosion resistance in applications not exposed to stress conditions. Such corrosion resistance is due to the inherent resistance of polyester resin to acid and alkali attack. However, when the pipe is subjected to heavy external loading, for example, by the weight of backfill placed over it when the pipe section is buried in the ground, crazing of the interior surface of the resin, often a gel coat, can occur, thereby exposing the filaments to acidic conditions.
More specifically, when a pipe section is subjected to such external loading, deformation of the pipe section is resisted by shear stresses which tend to concentrate along the inner-circumferential wall surface of the pipe. These shear stresses, in turn, can result in the resin crazing or cracking along this inner-circumferential surface. If excessive strain causes the resin to crack or craze, the glass filaments may be exposed to corrosive attack. When corrosive fluid, such as acid, attacks the glass filaments, penetration of the entire pipe wall structure by the acid can occur very rapidly and can result in mechanical failure of the entire composite structure. This cracking problem and resultant mechanical failure is particularly likely when the composite structure is used as sewer pipe. Bacteria and other microorganisms attack organic materials in domestic sewage to form acids capable of attacking the glass reinforcement filaments. Since the glass filaments provide strength to the composite structure, if these are weakened by corrosive attack, the entire pipe can fail.
Various attempts have been made in the past to overcome the potential failure problem of reinforced plastic pipe due to corrosive attack of the wall structure. Most of these attempts have concentrated on various improvements of a craze resistant interior surface gel coat lining to protect the glass fibers from acid attack. One known craze resistant gel coat is disclosed in U.S. Pat.No. 3,893,488. The present invention does not require a special inner lining or gel coat, but rather provides a reinforced plastic structure which throughout its entire thickness is designed to resist crazing under normal external stress conditions. Even if some cracking or crazing of the resin matrix results due to external stress on the structure, the resin matrix of the present invention has been found to inhibit propagation of such cracks. Since any cracks which do result do not increase in size or number by further exposure to acidic fluids, the glass filaments are not exposed to acid attack and retain their reinforcement ability.
Heretofore, the resin matrix in reinforced plastic pipe composites generally has been filled with a relatively inexpensive inert filler such as aluminum silicate clay. For example, U.S. Pat. No. 3,706,615 discloses the incorporation of additives such as sand, calcium carbonate generally, or clay in the resin and U.S. Pat. No. 3,406,724 discloses the incorporation of 20-85% of a filler such as quartz, chalk, kaolin, or rock flour in plastic sewer pipe. As will be seen hereinafter, the present invention is not intended to cover the incorporation of calcium carbonate generally as a filler. Rather, the present invention is directed to incorporating into the resin component a particulate substance having a specific particle size and selected from the group consisting of portland cement, marble dust or a mixture thereof. These substances are not merely inert fillers, but rather, their incorporation in the resin produces a structure exhibiting much greater acid resistance than prior art composite structures.